There have been two kinds of method for the manufacture of polyaspartic acid by polymerizing maleic acid and ammonia, i.e., a liquid phase polymerization method and a solid phase polymerization method, which is carried out in the presence or the absence of a solvent, respectively.
For a solid phase polymerization method, which generally does not require a solvent, derivatives of maleic acid are reacted with a source of ammonia and then heated directly for the polymerization. Because the polymerization is carried out in a solid phase using solid reacting materials, handling is not easy and foaming occurs inevitably. As a result, heat transfer efficiency of the process is low. Thus, the reaction temperature has to be increased and the reaction time has to be extended to overcome said problems associated with the method. However, higher the reaction temperature, more of the reaction mixture converts to a foamed solid, which impairs the heat transfer efficiency of the process, while the reaction temperature has to be increased and the reaction time has to be extended to achieve a more efficient reaction. Moreover, because the reacting materials are poorly admixed, molecular weight of the final product generally becomes to be smaller.
For a liquid phase polymerization method using a solvent, degree of foaming varies depending on the characteristics of the solvent used. When a polar solvent without active hydrogen is used, foaming can be prevented but the product tends to have a higher level of heat degradation, low quality and poor color. If a weakly polar solvent or a nonpolar solvent that is different from said polar solvent is used, the degree of foaming can be lowered but cannot be prevented. In addition, due to poor stability maleic acid salts are decomposed by heat during the polymerization process. As a result, quality of the synthetic product becomes deteriorated.
Polyaspartic acid is a polymer produced from the hydrolysis of polysuccinimide and is soluble in water. As a biodegradable polymer, it can be applied to various fields. Particularly, it can be used as a builder for detergent, a water-treatment agent for boiler and cooling water, a product adopted for agricultural use, a personal product such as shampoo, and a vehicle for drug delivery in pharmaceuticals and agricultural chemicals. Typically, polyaspartic acid is prepared by hydrolyzing polysuccinimide that is obtained from condensation polymerization of dibasic acid such as maleic acid and fumaric acid with ammonia or from condensation polymerization of L-aspartic acid or DL-aspartic acid. A known process for producing polyaspartic acid includes the following.
U.S. Pat. No. 5,466,779 discloses a method of preparing polyaspartic acid, which comprises the steps of reacting ammonia with maleic anhydride while maintaining the temperature at 100° C. to 110° C. to give maleamic acid, polymerizing the obtained maleamic acid to polysuccinimide by heating the acid to a reaction temperature of from 180° C. to 240° C., and hydrolyzing the obtained polysuccinimide with caustic soda or other corresponding base to produce polyaspartic acid.
U.S. Pat. No. 5,373,088 discloses a method of producing polyaspartic acid, which comprises the steps of reacting maleic acid and ammonia to produce an ammonium maleate, polymerizing the obtained ammonium maleate at temperature of from 220° C. to 240° C. for 7 to 10 hours to produce a polysuccinimide, and hydrolyzing the obtained polysuccinimide to produce polyaspartic acid.
U.S. Pat. No. 5,508,434 discloses a method of producing polyaspartic acid, which comprises the steps of heat-polymerizing aspartic acid in the presence of a sulfur-containing dehydrating agent to obtain polysuccinimide with relatively high molecular weight, and hydrolyzing the obtained polysuccinimide to produce polyaspartic acid.
U.S. Pat. No. 5,371,180 discloses a method of producing polyaspartic acid, which comprises the steps of polymerizing fumaric acid or maleic acid with a ammonia-containing agent including urea, carbamic acid, ammonium carbonate, ammonium bicarbonate and diammonium carbonate in an amount of from 0.95 to 1.05 mol per one mole of dibasic acid at the temperature of from 160° C. to 220° C. to give polysuccinimide, and hydrolyzing the obtained polysuccinimide with caustic soda or other corresponding base to produce polyaspartic acid.
U.S. Pat. No. 5,714,588 discloses a method of producing polyaspartic acid, which comprises the steps of heat-polymerizing at the temperature of from 150° C. to 220° C. aspartic acid, fumaric acid, maleic acid or maleic anhydride with ammonia, that is specifically selected in consideration of each organic acid used, in the presence of various kinds of solvents to produce polyaspartic acid.
On the other hand, Korean Laid-open Patent Application No. 2003-0073994 describes a method of producing polyaspartic acid comprising steps of heat-polymerizing dibasic acid with ammonia in the presence of silicone solvent to give polysuccinimide and hydrolyzing the obtained polysuccinimide to produce polyaspartic acid.
When a solid phase polymerization is employed among previous methods, the reaction temperature has to be increased and the reaction time has to be extended due to the poor heat-transfer efficiency. As a result, energy consumption is high and the obtained polymer resin is highly viscous so that water produced according to the proceeding of the reaction is not removed effectively, causing a foaming phenomenon. Accordingly, problems arise including that an abnormally large manufacturing unit has to be constructed, etc. Moreover, as the unreacted materials cannot be effectively admixed, the polymer obtained has a weight average molecular weight of only from 3000 to 5000.
Meanwhile, in a liquid phase polymerization method, degree of foaming varies depending on characteristics of the solvent used. When a polar solvent without active hydrogen is used, foaming can be prevented but the product tends to have a higher level of heat degradation, low quality and poor color. If a weakly polar solvent or a nonpolar solvent is used, the degree of foaming can be lowered but cannot be prevented. In addition, due to poor stability maleic acid salts are decomposed by heat during the polymerization process. As a result, quality of the synthetic product becomes deteriorated.
Under the circumstances, there is a need for developing a technology of preparing a polymer product with high quality while foaming in reacting materials is prevented.